This invention relates to a method and apparatus for determining the impact time of a valve armature of a solenoid valve, such as is used, for example, in a fuel injector of a vehicle internal combustion engine.
In fuel injection technology, it is important to determine the opening and closing times of the injection valves as accurately as possible in order to maintain given limit curves from one injection to the next without any control, for example, to minimize exhaust emissions. If the respective opening and closing times of the injection valve are known, the fuel quantity injected during the open phase can be determined from the sequence of the internal movements of the injector.
The opening and closing times of the injection valve are, in turn, determined from the armature impact in energizing and de-energizing of the solenoid. In the operating sequence of the injectors, the two impact times are affected by the spring biasing of the valve armature; and fluctuations of the opening and closing behavior of the injection valves caused by spring tolerances, spring holding and mechanical mounting tolerances can be compensated by suitably regulating the injection technique.
Measuring methods for determining the energizing impact time are described, for example, in German Patent Documents DE 42 37 706 A1 and DE 37 30 523 A1 in connection with the start of the injection. Energizing measuring methods will therefore not be discussed in detail in the following.
Concerning the armature impact time after de-energizing of the solenoid (that is, at the actual end of injection), German Patent Document DE 37 30 523 A1 discloses an arrangement in which, after the actuating current is switched off by means of the magnet winding, the induction voltage caused by the movement of the solenoid armature in the magnet winding is amplified to a detectible signal level by means of an external energy source, in order to better monitor the switching times which are thus indicated more clearly.
Although the latter technique clearly indicates the switch-off time of the actuating current during energizing, it nevertheless has the disadvantage that a signal which is quite weak must be amplified. Particularly during de-energizing, such a signal is extremely indistinct and hard to determine, because the magnet coil must be completely de-energized in order to cause the magnet armature to drop. In practice, this is achieved by feeding a high extinguishing voltage. However, since the coil of the solenoid is not energized during the actual travel phase, the magnetic circuit is demagnetized. Thus, no magnetic field is built up in the magnet coil, and no magnetic interactions occur between the positional and motional relationship of the valve armature and the magnet coil. As a result, no induced voltage is available to detect the impact.
It is therefore an object of the present invention to provide a method and apparatus which achieves a clear determination of the armature impact time after the de-energizing, by technically simple means, at a reasonable cost.
This object is achieved by the method and apparatus according to the invention, in which current is built up separately in the magnet coil after an armature adhesion point is exceeded (that is, after the start of the armature travel phase). This measuring current must be large enough to create a magnetic field in the magnet coil which is sufficient to generate a recognizable induction voltage when changes occur. However, at the same time, this measuring current should also be low enough that the magnetic field which it generates will not hinder the armature's downward movement.
An important advantage of the technique according to the invention is that it eliminates the need for quantitative processing of a signal which is hard to interpret. Rather, the induction voltage signal itself, used to determine the impact time, is qualitatively much more clearly recognizable. In addition, such a reinforcement of the cause takes place completely independently of a possible subsequent processing of the induction voltage signal.
By virtue of the clearly determinable de-energizing armature impact time according to the invention, and with the determination of the energizing armature impact time known from the state of the art, adjustment of the armature spring biasing is unnecessary. Since therefore the injection valves no longer have to be calibrated, their handling requires lower costs during manufacturing and exchange.
Furthermore, in the case of injection valves, because of the clearly identifiable closing time signal obtained according to the invention, devices for amplifying the signal, which require high cost wiring, are unnecessary. For this reason, the injection valves and the solenoid contained therein may have a simpler and smaller construction.
According to the invention, the measuring current built up in the magnet coil during the travel phase of the valve armature is maintained at a constant value in order to obtain a voltage signal which is induced exclusively by the armature drop-out movement.
A clearly determinable induction voltage signal is therefore generated so that reading and/or recognition errors, which result from a weak or insufficiently pronounced signal, can be avoided. Furthermore, signal voltage values suitable for regulating purposes, may also be reached inductively as a result of the armature's downward movement in the magnet coil, without any additional signal processing or signal amplification. In addition to the fact that previously required signal amplification devices are thus no longer necessary, which simplifies the regulator expenditures, the method and apparatus according to the invention also provide a distortionless signal course, on which no additional time-related or qualitative interfering influences are imposed.
An interference-free motion signal of the magnet armature obtained in this manner, with a distinctive signal during the armature impact that remains clearly recognizable over many injections, permits a preferred embodiment of the invention in which the measuring current connected during the armature travel phase is held to a constant value. The purpose of keeping the current constant is to compensate for magnetic field changes in the magnet coil which result from fluctuations of the magnetic field exciting current (that is, of the measuring current).
It is a basic advantage of such measuring current regulation that mutual compensation of voltage induced in the magnet coil and the auxiliary voltage which drives the measuring current (and is essentially opposite), can be avoided. This ensures that an energetically constant magnetic field exists in the solenoid valve coil over the whole armature downward travel phase, and thus only those magnetic field changes which are caused by the position or the motion of the armature determine the signal course.
In order to control the measuring current to a desired constant value, according to another advantageous embodiment of the invention, depending on the respective requirements, both positive and negative auxiliary voltages are alternately added to the magnet voltage in order to ensure the controllability of the current, irrespective of the direction and amount of the induced voltage.
The invention can expediently be practiced using known current/voltage regulators. However, such regulators can only control a constant measuring current if a corresponding auxiliary voltage signal is present as the regulator input quantity. Particularly in an embodiment of the method according to the invention in which the measuring current is controlled to a constant value, therefore, it is necessary for problem-free regulator operation, that the maximum value of the positive or negative adjustable auxiliary voltage which can be fed to the magnet coil be larger than the voltage induced in the magnet coil during the travel phase. For this purpose, the auxiliary voltage may analogously be fed to the magnet coil, so that a particularly low-cost regulator device may be used.
It is also advantageous to switch a timed auxiliary voltage to the magnet coil in order to keep the power loss of the end stage as low as possible.
A suitable device for implementing the invention therefore provides a controllable auxiliary voltage source, in the form of an analog or digital computer, which is series-connected with the magnet coil. A particularly simple and low cost arrangement is obtained if the device provided for the rapid de-energizing of the magnet coil is also used to generate the controllable auxiliary voltage, which can be fed to the solenoid valve coil by means of components which exist in the device anyhow.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.